U.S. patent application number 10/551065 was filed with the patent office on 2006-11-23 for method for cement bond evaluation in boreholes.
Invention is credited to Karine Guibert, Virginie Schoepf.
Application Number | 20060262644 10/551065 |
Document ID | / |
Family ID | 32842872 |
Filed Date | 2006-11-23 |
United States Patent
Application |
20060262644 |
Kind Code |
A1 |
Schoepf; Virginie ; et
al. |
November 23, 2006 |
Method for cement bond evaluation in boreholes
Abstract
A method for determining components of a tube casing surrounding
a tube uses amplitude measurements of an acoustic signal wavefront
emitted inside the tube. The acoustic signal wavefront is affected
by the components of the tube casing while propagating in the tube.
Various parameters such as an attenuation A(E) of the amplitude
inside the tube and a coupling amplitude E.sub.o may be determined.
Theses parameters may be inserted in equations that are inverted to
obtain a percentage of cement bonded .phi..sub.C in the tube
casing, a percentage of cement de-bonded .phi..sub.D in the tube
casing and a percentage of liquid .phi..sub.W in the tube
casing.
Inventors: |
Schoepf; Virginie;
(Aberdeen, FR) ; Guibert; Karine; (Aberdeen,
GB) |
Correspondence
Address: |
SCHLUMBERGER OILFIELD SERVICES
200 GILLINGHAM LANE
MD 200-9
SUGAR LAND
TX
77478
US
|
Family ID: |
32842872 |
Appl. No.: |
10/551065 |
Filed: |
March 22, 2004 |
PCT Filed: |
March 22, 2004 |
PCT NO: |
PCT/EP04/04918 |
371 Date: |
July 28, 2006 |
Current U.S.
Class: |
367/35 |
Current CPC
Class: |
G01V 1/50 20130101; G01N
29/4472 20130101; G01N 2291/0231 20130101; G01N 2291/2636 20130101;
G01N 29/11 20130101; G01N 29/52 20130101; G01N 2291/103 20130101;
G01N 2291/044 20130101 |
Class at
Publication: |
367/035 |
International
Class: |
G01V 1/00 20060101
G01V001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 3, 2003 |
EP |
03290833.7 |
Claims
1. A method for determining components of a tube casing surrounding
a tube, using amplitude measurements of an acoustic signal
wavefront emitted inside the tube, the acoustic signal wavefront
being affected by the components of the tube casing while
propagating in the tube the method comprising Determining an
attenuation A(E) of the amplitude inside the tube, Determining a
percentage of cement bonded .phi..sub.C in the tube casing, a
percentage of cement de-bonded .phi..sub.D in the tube casing and a
percentage of liquid .phi..sub.W in the tube casing, by inverting
the following equations:
A(E)=.phi..sub.C*A(E.sub.fc)+.phi..sub.W*A(E.sub.fp)+.phi..sub.D*A(E.sub.-
fd)
logE.sub.P1=.phi..sub.C*logE.sub.P1,fc+.phi..sub.W*logE.sub.P1,fp+.ph-
i..sub.D*logE.sub.P1,fd .phi..sub.C+.phi..sub.W+.phi..sub.D=1
wherein E.sub.P1 is the amplitude measured at a location P1,
E.sub.P1,fp is the amplitude at the location P1 for a free-pipe
tube casing, E.sub.P1,fc is the amplitude at the location P1 for a
fully cemented tube casing, E.sub.P1,fd is the amplitude at the
location P1 for a fully de-bonded tube casing, and A(E.sub.fp),
A(E.sub.fc), A(E.sub.fd) being the attenuations of amplitude in
case the tube casing is respectively free-pipe, fully cemented, and
fully de-bonded.
2. A method for determining components of a tube casing surrounding
a tube, using amplitude measurements of an acoustic signal
wavefront emitted inside the tube, the acoustic signal wavefront
being affected by the components of the tube casing while
propagating in the tube, the method comprising Determining a
coupling Amplitude E.sub.o by extrapolating amplitude measurements
made at various locations in the tube to a location of a source of
the acoustic signal, Determining a percentage of cement bonded
.phi..sub.C in the tube casing, a percentage of cement de-bonded
.phi..sub.D in the tube casing and a percentage of liquid
.phi..sub.W in the tube casing, by inverting the following
equations:
logE.sub.P1=.phi..sub.C*logE.sub.P1,fc+.phi..sub.W*logE.sub.P1,fp+.phi..s-
ub.D*logE.sub.P1,fd
logE.sub.0=.phi..sub.C*logE.sub.0,fc+.phi..sub.W*logE.sub.0,fp+.phi..sub.-
D*logE.sub.0,fd .phi..sub.C+.phi..sub.W+.phi..sub.D=1 wherein
E.sub.P1 is the amplitude measured at a location P1, E.sub.P1,fp is
the amplitude at the location P1 for a free-pipe tube casing,
E.sub.P1,fc is the amplitude at the location P1 for a fully
cemented tube casing, E.sub.P1,fd is the amplitude at the location
P1 for a fully de-bonded tube casing, E.sub.0,fp is the coupling
amplitude for a free-pipe tube casing, E.sub.0,fc is the coupling
amplitude for a fully cemented tube casing, E.sub.0,fd is the
coupling amplitude for a fully de-bonded tube casing.
3. A method for determining components of a tube casing surrounding
a tube, using amplitude measurements of an acoustic signal
wavefront emitted inside the tube, the acoustic signal wavefront
being affected by the components of the tube casing while
propagating in the tube, the method comprising Determining an
attenuation A(E) of the amplitude inside the tube, Determining a
coupling Amplitude E.sub.o by extrapolating amplitude measurements
made at various locations in the tube to a location of a source of
the acoustic signal, Determining a percentage of cement bonded
.phi..sub.C in the tube casing, a percentage of cement de-bonded
.phi..sub.D in the tube casing and a percentage of liquid
.phi..sub.W in the tube casing, by inverting the following
equations:
logE.sub.0=.phi..sub.C*logE.sub.0,fc+.phi..sub.W*logE.sub.0,fp+.phi..sub.-
D*logE.sub.0,fd
A(E)=.phi..sub.C*A(E.sub.fc)+.phi..sub.W*A(E.sub.fp)+.phi..sub.D*A(E.sub.-
fd) .phi..sub.C+.phi..sub.W+.phi..sub.O=1 wherein A(E.sub.fp),
A(E.sub.fc), A(E.sub.fd) correspond respectively to the
attenuations of amplitude in case the tube casing is free-pipe,
fully cemented, and fully debonded, and E.sub.0,fp is the coupling
amplitude for a free pipe tube casing, E.sub.0,fc is the coupling
amplitude for a fully cemented tube casing, E.sub.0,fd is the
coupling amplitude for a fully de-bonded tube casing.
4. The method according to anyone of claims 2 to 3, wherein
determining the coupling amplitude E.sub.o further comprises
Defining a linear function E(X)=A*X+E.sub.o in which an amplitude
E(X) of an acoustic signal wavefront measured at a location X in
the tube is subject to an attenuation A, and Calculating E(X)=E(0)
for a location where X=0, corresponding to a source of the acoustic
signal in the tube.
Description
BACKGROUND OF THE INVENTION
[0001] The invention generally relates to an apparatus and method
for acoustically investigating a tube casing.
[0002] One example of a tube casing is a wall of a borehole
penetrating an earth formation. Such a wall may comprise an annular
space filed with set cement. After the cement has set in the
annular space of the casing it is common practice to use acoustic
non-destructive testing methods to evaluate its integrity. This
evaluation is of prime importance since the cement must guarantee
zonal isolation between different formations in order to avoid flow
of fluids from the formations (water, gas, oil) through the annular
space of the casing. FIG. 1A contains an example of a vertical
cross section of a borehole 100 in a formation 101. The borehole
comprises at least one tube 102 that may for example be made out of
steel. A casing 103 surrounds the tube 102 and provides an annular
space ideally filled with cement. FIG. 1B illustrates a horizontal
cross section of the borehole 100.
[0003] A common cause for loss of isolation from the casing is the
presence of a channel in the casing, i.e., an angular section of
the annulus that is filled by a liquid instead of cement. Another
cause for loss of isolation may be the presence of a microscopic
annular space (microannulus) between Ie tube and the cement. In
both cases a risk occurs that fluids flow along the cement casing
and generate problems of fluid migration, or even worse a collapse
of the borehole. It is therefore essential to be able to detect
both causes, i.e., channel and microannulus, and remedy to their
presence in the casing.
[0004] One possible way of remedy when a channel is detected in the
casing, is to shoot a hole through the casing and to inject a
cement slurry through the hole into the channel to seal the
channel. This action is commonly referred to as "squeeze". A
squeeze is expensive to perform.
[0005] Unfortunately it may in some cases of evaluation happen that
a channel is detected although no channel is present in the casing,
leading to a useless squeeze and unnecessary expenses. One reason
for such a "false-positive" detection may be the presence of a
microannulus. Conventional detection and evaluation methods appear
to be mislead by the presence of a microannulus and to deliver a
value for the percentage of the annulus filled by cement that is
lower than reality.
SUMMARY OF THE INVENTION
[0006] In a first aspect the invention provides a method for
determining components of a tube casing surrounding a tube, using
amplitude measurements of an acoustic signal wavefront emitted
inside the tube. The acoustic signal wavefront is affected by the
components of the tube casing while propagating in the tube. An
attenuation A(E) of the amplitude inside the tube is determined. A
percentage of cement bonded .phi..sub.C in the tube casing, a
percentage of cement de-bonded .phi..sub.D in the tube casing and a
percentage of liquid .phi..sub.W in the tube casing, are determined
by inverting the following equations:
A(E)=.phi..sub.C*A(E.sub.fc)+.phi..sub.W*A(E.sub.fp)+.phi..sub.D*A(E.sub.-
fd)
logE.sub.P1=.phi..sub.C*logE.sub.P1,fc+.phi..sub.W*logE.sub.P1,fp+.ph-
i..sub.D*logE.sub.P1,fd .phi..sub.C+.phi..sub.W+.phi..sub.D=1
wherein E.sub.P1 is the amplitude measured at a location P1,
E.sub.P1,fp is the amplitude at the location P1 for a free-pipe
tube casing, E.sub.P1,fc is the amplitude at the location P1 for a
fully cemented tube casing, E.sub.P1,fd is the amplitude at the
location P1 for a fully de-bonded tube casing, and A(E.sub.fp),
A(E.sub.fc), A(E.sub.fd) being the attenuations of amplitude in
case the tube casing is respectively free-pipe, fully cemented, and
fully de-bonded.
[0007] In a second aspect the invention provides a method for
determining components of a tube casing surrounding a tube, using
amplitude measurements of an acoustic signal wavefront emitted
inside the tube. The acoustic signal wavefront is affected by the
components of the tube casing while propagating in the tube. A
coupling Amplitude E.sub.o is determined by extrapolating amplitude
measurements made at various locations in the tube to a location of
a source of the acoustic signal. A percentage of cement bonded
.phi..sub.C in the tube casing, a percentage of cement de-bonded
.phi..sub.D in the tube casing and a percentage of liquid
.phi..sub.W in the tube casing, are determined by inverting the
following equations:
logE.sub.P1=.phi..sub.C*logE.sub.P1,fc+.phi..sub.W*logE.sub.P1,fp+.phi..s-
ub.D*logE.sub.P1,fd
logE.sub.0=.phi..sub.C*logE.sub.0,fc+.phi..sub.W*logE.sub.0,fp+.phi..sub.-
D*logE.sub.0,fd .phi..sub.C+.phi..sub.W+.phi..sub.D=1 wherein
E.sub.P1 is the amplitude measured at a location P1, E.sub.P1,fp is
the amplitude at the location P1 for a free-pipe tube casing,
E.sub.P1,fc is the amplitude at the location P1 for a fully
cemented tube casing, E.sub.P1,fd is the amplitude at the location
P1 for a fully de-bonded tube casing, E.sub.0,fp is the coupling
amplitude for a free-pipe tube casing, E.sub.0,fc is the coupling
amplitude for a fully cemented tube casing, E.sub.0,fd is the
coupling amplitude for a fully de-bonded tube casing.
[0008] In a third aspect the invention provides a method for
determining components of a tube casing surrounding a tube, using
amplitude measurements of an acoustic signal wavefront emitted
inside the tube. The acoustic signal wavefront is affected by the
components of the tube casing while propagating in the tube. An
attenuation A(E) of the amplitude inside the tube, is determined
and a coupling Amplitude E.sub.o is found by extrapolating
amplitude measurements made at various locations in the tube to a
location of a source of the acoustic signal. A percentage of cement
bonded .phi..sub.C in the tube casing, a percentage of cement
de-bonded .phi..sub.D in the tube casing and a percentage of liquid
.phi..sub.W in the tube casing, are dertmined by inverting the
following equations:
logE.sub.0=.phi..sub.C*logE.sub.0,fc+.phi..sub.W*logE.sub.0,fp+.phi..sub.-
D*logE.sub.0,fd
A(E)=.phi..sub.C*A(E.sub.fc)+.phi..sub.W*A(E.sub.fp)+.phi..sub.D*A(E.sub.-
fd) .phi..sub.C+.phi..sub.W+.phi..sub.O=1 wherein A(E.sub.fp),
A(E.sub.fc), A(E.sub.fd) correspond respectively to the
attenuations of amplitude in case the tube casing is free-pipe,
fully cemented, and fully debonded, and E.sub.0,fp is the coupling
amplitude for a free pipe tube casing, E.sub.0,fd is the coupling
amplitude for a fully cemented tube casing, E.sub.0,fd is the
coupling amplitude for a fully de-bonded tube casing.
[0009] Preferably the coupling amplitude E.sub.o determined by
defining a linear function E(X)=A*X+E.sub.o in which an amplitude
E(X) of an acoustic signal wavefront measured at a location X in
the tube is subject to an attenuation A, and calculating E(X)=E(0)
for a location where X=0, corresponding to a source of the acoustic
signal in the tube.
BRIEF DESCRIPTION OF THE FIGURES
[0010] The invention will now be described in greater detail with
reference to the accompanying drawings, in which:
[0011] FIG. 1A contains a schematic vertical cross section of a
borehole according to prior art,
[0012] FIG. 1B contains a schematic horizontal cross section of a
borehole according to prior art,
[0013] FIG. 2A shows a schematic representation of a cement bond
evaluating device according to prior art,
[0014] FIG. 2B contains a representation of an acoustic path
between a source and receivers of an acoustic signal,
[0015] FIG. 3 illustrates an example of a measured amplitude
signal,
[0016] FIG. 4 illustrates an example of an annular space comprising
cement and liquid, and
[0017] FIG. 5 contains a flow chart illustrating an example
embodiment of a method according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] Same references will be used to reference same elements in
the Figures throughout the description.
Measurement Set-Up
[0019] FIG. 2 contains a schematic representation of a Cement Bond
Evaluating Device 200 (CBED) inserted in a borehole delimited by
the tube 102 and the casing 103, formed in the formation 101. The
CBED 200 comprises an Acoustic Signal Source (ASS) 201, located at
a first location X.sub.0, and Acoustic Signal Receivers (ASR) 202
and 203 located respectively at a second location X.sub.1 and a
third location X.sub.2.
[0020] As an example the ASS 201 may be a ceramic piezo-electrical
transducer. The distances separating the ASS 201 from the ASR 202
and 203 may for example be respectively 90 cm and 150 cm. The ASS
201 emits acoustic pulses having a duration of 50 .mu.s with a rate
of 10 to 60 pulses per second. A typical main frequency of the
acoustic signal may be 20 kHz. Other frequencies may be used in
other examples.
[0021] Every pulse emitted by the ASS 201 generates a wavefront
that propagates through the tube 102. After interacting with the
tube 102 and the casing 103, the acoustic wave reaches the ASRs 202
and 203 where the time dependent amplitudes are measured. The
measured amplitude is in fact the result of a composite wave that
corresponds to an addition of waves that have travelled different
paths (tube, casing . . . ). The measured amplitude reflects an
acoustic impedance and an acoustic coupling that exists between the
tube and a content of the adjacent casing.
[0022] It is convenient to measure amplitudes of a determined peak
as the determined peak may more easily be identified as it
propagates with the wavefront from the ASR 202 to the ASR 203.
[0023] FIG. 3 shows an example curve of time dependent amplitude
measurements that may for example be taken at the ASR 202. The
curve goes through a number of peaks E.sub.1, E.sub.2, E.sub.3 . .
. wherein E.sub.1 appears to be the first peak after a transit time
corresponding to time lapsed during propagation from the source to
the ASR 202.
Attenuation of Amplitude
[0024] An attenuation of the amplitude of the acoustic signal
during propagation of the wavefront through the tube in
longitudinal direction may be determined by evaluating measurements
taken with two separate ASRs.
[0025] For example we may consider two ASRs, the first of which is
2.4 ft (0.73152 m) away from the source, the second of which is 3.4
ft (1.03632 m) away from the source, and calculate the attenuation
as follows: A = - 20 L .times. log 10 .times. E .times. .times. 1 1
E .times. .times. 1 2 ##EQU1## wherein: A is the attenuation factor
in dB/ft, E1.sub.1 is the amplitude of the E.sub.1 peak at the
first ASR (2.4 ft), E1.sub.2 is the amplitude of the E.sub.1 peak
at the second ASR (3.4 ft), L is the distance between the first ASR
and the second ASR. Interpretation of Measured Acoustic
Amplitudes
[0026] It has previously been stated that the measured amplitude
reflects an acoustic impedance and an acoustic coupling that exists
between the tube and a content of the adjacent casing. The
following discussion will be based on consideration of the peak E1,
as an example. It is understood that any other peak or energy
computed inside a temporal window taken from the curve of amplitude
measurement may be considered instead as appropriate.
[0027] There are generally 3 extreme cases that may occur: [0028]
The annulus is totally filled with water. This case is known as
free-pipe (fp). In this case the peak E.sub.1 is subject to a
relatively small attenuation. A tube's vibration resulting from the
acoustic signal will propagate with a minimum loss of intensity.
[0029] The annulus is entirely cemented. This case is known as
fully cemented (fc). In this case the peak E.sub.1 is subject to a
relatively strong attenuation. [0030] The annulus is entirely
cemented except for a microscopic interval between the cement and
the periphery of the tube. This case is known as fully de-bonded
(fd). The attenuation has a value comprised between the values
known from the previous cases, i.e. free-pipe and fully
cemented.
[0031] Various methods exist in prior art to establish a
quantitative evaluation of the components present in the
casing.
[0032] One example method described in Grosmangin et al., A Sonic
Method for Analysing the quality of Cementation of Borehole
Casings, J Pet. Tech, Trans., AIME, 222, February 1961, allows to
evaluate a percentage of cement in the casing by making use of a
parameter called Bond Index (BI) as follows: BI = log .function. (
E .times. .times. 1 measured ) - log .function. ( E .times. .times.
1 fp ) log .function. ( E .times. .times. 1 fc ) - log .function. (
E .times. .times. 1 fp ) ##EQU2## wherein: E1.sub.measured is the
measured amplitude, E1.sub.fp is the value of the amplitude in the
free pipe case, and E1.sub.fc is the value of the amplitude in the
fully cemented case.
[0033] The BI is mainly adapted to determine a percentage of cement
bonded in the tube casing (.phi..sub.C) and a percentage of liquid
in the tube casing (.phi..sub.W). An example of such a situation is
shown in FIG. 4, in which a schematic cross section of a casing 400
surrounding a tube 403, containing a percentage of cement 401 and a
percentage of liquid 402 is shown.
[0034] The BI may however be subject to false interpretation when
in presence of a microannulus. A percentage of the microannulus in
the casing is referenced as .phi..sub.D.
[0035] Another example method described in Leslie H. D., Selliers
J. de, Pittman D. J., Coupling and Attenuation: A New Measurement
Pair in Cement Bond Logging, SPE 16207, 1984, makes use of a
Coupling Amplitude (CA) to take into account the effect of a
microannulus on the measured amplitude. In this example, the
amplitude equals a value of the amplitude for an effective spacing
equal to zero, wherein the effective spacing is the source-receiver
distance where the curve of fully bonded cement intersects the
curve of fully debonded cement in the plot peak
amplitude/source-receiver spacing. Also, a Cement Index (CI) is
defined in addition to the Bond Index. The CI allows to determine
the percentage of cement .phi..sub.C in the casing even if the
cement is separated from the tube by a microannulus.
[0036] In the further description of embodiments of the present
invention, the Coupling Amplitude is defined as being a value
E.sub.0 of the amplitude at a location of the tube corresponding to
the source of the acoustic signal. The value E.sub.0 of the
Coupling Amplitude may be obtained by considering a linear function
representing a value of the amplitude as a function of a distance
to the source of acoustic signal. The value of the linear function
at a zero distance is the value E.sub.0 of the Coupling
Amplitude.
[0037] Considering for example two measurements E1.sub.X1 and
E1.sub.X2 of the amplitude of the peak E1 made at locations X1 and
X2, we have following equations defining a linear function E(X),
wherein A represents the attenuation of the amplitude:
E(X1)=E1.sub.X1=A*X1C+E.sub.0 (1) E(X2)=E1.sub.X2=A*X2C+E.sub.0 (2)
wherein X1C and X2C are corrected values of X1 and X2 corresponding
to the distance between receiver and source minus a path that is
the sum of an acoustic path between source and casing and an
acoustic path between casing and receiver, according to Snell's law
and as illustrated in FIG. 2B. Accordingly X1C and X2C are
expressed as follows: X1C=X1-2*h*tan(.alpha.)
X2C=X2-2*h*tan(.alpha.).
[0038] The parameter h corresponds to an orthogonal distance
between the Cement Bond Evaluating Device and the casing, also
known as the standoff of the tool. The angle .alpha. may be
obtained as follows: .alpha.=a sin(vfluid/vcasing) wherein vfluid
and vcasing are known acoustic velocities of fluid surrounding the
Cement Bond Evaluating Device and of the casing.
[0039] E.sub.0 is thus obtained by resolving equations (1) and (2)
as follows: E 0 = X .times. .times. 2 .times. C * E .times. .times.
1 X .times. .times. 1 - X .times. .times. 1 .times. C * E .times.
.times. 1 X .times. .times. 2 X .times. .times. 2 .times. C - X
.times. .times. 1 .times. C ##EQU3##
[0040] According to measurement results obtained in the frame of
the present invention it was found that the respective percentages
.phi..sub.C, .phi..sub.W and .phi..sub.D may be related in the
following 4 equations (3), (4), (5) and (6), involving measured and
calculated values of the attenuation A(E) of an acoustic signal and
the coupling amplitude E.sub.0 for this signal:
A(E)=.phi..sub.C*A(E.sub.fc)+.phi..sub.W*A(E.sub.fp)+.phi..sub.D*A(E.sub.-
fd) (3)
logE.sub.P1=.phi..sub.C*logE.sub.P1,fc+.phi..sub.W*logE.sub.P1,f-
p+.phi..sub.D*logE.sub.P1,fd (4)
logE.sub.0=.phi..sub.C*logE.sub.0,fc+.phi..sub.W*logE.sub.0,fp+.phi..sub.-
D*logE.sub.0,fd (5) .phi..sub.C+.phi..sub.W+.phi..sub.D=1 (6)
[0041] In equation (3), A(E.sub.fc), A(E.sub.fp) and A(E.sub.fd)
are attenuations of the amplitude in extreme cases corresponding
respectively to the fully cemented, the free-pipe and the fully
de-bonded cases.
[0042] In equation (4), E.sub.P1 is the amplitude of the acoustic
signal measured at a location P1. E.sub.P1,fc', E.sub.P1,fp and
E.sub.P1,fd are amplitudes of the acoustic signal at the location
P1 in extreme cases corresponding respectively to the fully
cemented, the free-pipe and the fully de-bonded cases.
[0043] In equation (5), E.sub.0,fc, E.sub.0,fp and E.sub.0,fd are
coupling amplitudes in extreme cases corresponding respectively to
the fully cemented, the free-pipe and the fully de-bonded
cases.
[0044] Equation (6) reflects that the sum of all percentages should
be 100%.
[0045] Hence it is possible to obtain the percentages .phi..sub.C,
.phi..sub.W and .phi..sub.D by inverting any combination of 3
equations from the 4 equations. For example the percentages may be
obtained by inverting the following systems of equations:
(3), (5) and (6);
(4), (5) and (6);
(3), (4) and (6).
[0046] Hence it is possible, using the invention, to obtain the
three percentages of cement, liquid and microannulus, based on
values of the coupling amplitude and/or of the attenuation in the
extreme situations of the fully cemented, free-pipe, and fully
de-bonded cases.
[0047] FIG. 5 contains a flowchart illustrating an example of a
method in which the system of equations (3), (5) and (6) is
inverted.
[0048] In box 500 the attenuations for the extreme cases of
free-pipe, fully cemented and fully de-bonded are determined. These
attenuations have to be determined once only for a given set of
borehole conditions defined by pressure and temperature and may be
re-used each time when a new determining of components of the tube
is made with the given set of borehole conditions, since these
attenuations have constant values. These attenuations may for
example be determined by means of measurements performed on a
prepared tube outside of the borehole.
[0049] In box 501 coupling amplitudes for the extreme cases of
free-pipe, fully cemented and fully de-bonded are determined. These
coupling amplitudes have to be determined once only and may be
re-used each time a new determining of components of the tube is
made, since these coupling amplitudes have constant values. These
coupling amplitudes may for example be determined by means of
measurements performed on a prepared tube outside of the
borehole.
[0050] In box 502 acoustic measurements are performed to determine
components of the tube casing inside the borehole. These
measurements are used to compute the attenuation A(E) in box 503
and the coupling amplitude E.sub.o in box 504.
[0051] At this point we have obtained the parameters for equations
(3), (5) and (6). The equations are inverted in box 505 using
well-known solving algorithms and as a result values for the
percentages .phi..sub.C, .phi..sub.W, .phi..sub.D are obtained.
[0052] Having described preferred embodiments of the invention with
reference to the accompanying drawings, it is to be understood that
the invention is not limited to these precise embodiments and that
various changes and modifications could be effected therein by a
person skilled in the art without departing from the spirit or
scope of the invention as defined in the appended claims. In
particular the invention is not limited to be used to evaluate
components of a cemented tube casing, and may for example well
apply to other materials used in casings such as resin or any other
material that could replace cement.
Notations Used Throughout the Description
[0053] E.sub.0 Coupling Amplitude at a location of an acoustic
signal emitter location [0054] .phi..sub.C Percentage of cement
bonded in tube casing [0055] .phi..sub.D Percentage of cement
de-bonded in tube casing [0056] .phi..sub.W Percentage of liquid in
tube casing [0057] BI Bond Index [0058] E.sub.fp Free-pipe
Amplitude [0059] E.sub.fc Fully cemented Amplitude [0060] E.sub.fd
Fully de-bonded Amplitude [0061] A(E Attenuation of Amplitude E
[0062] CI Cement Index [0063] ASS Acoustic Signal Source [0064] ASR
Acoustic Signal Receiver
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